Fluidized bed reactor for production of high purity silicon

ABSTRACT

Methods and apparatus for the production of high purity silicon including a fluidized bed reactor with one or more protective layers deposited on an inside surface of the fluidized bed reactor. The protective layer may be resistant to corrosion by fluidizing gases and silicon-bearing gases.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of now pending U.S. patentapplication Ser. No. 12/903,994, entitled FLUIDIZED BED REACTOR FORPRODUCTION OF HIGH PURITY SILICON, filed on Oct. 13, 2010, which is adivisional of U.S. patent application Ser. No. 12/393,852, entitledFLUIDIZED BED REACTOR FOR PRODUCTION OF HIGH PURITY SILICON, filed onFeb. 26, 2009, each of which is fully incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to methods for producing high purityelectronic grade silicon. More particularly, this disclosure relates tomethods for producing high purity silicon beads by chemical vapordeposition (CVD) of a silicon-bearing gas on seed particles bydecomposition in a fluidized bed reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows one embodiment of a process for the purification ofsilicon.

FIG. 2 shows one embodiment of a fluidized bed reactor used for thepurification of silicon.

FIG. 3 shows a close-up cross section of one embodiment of a fluidizedbed reactor as disclosed herein.

DETAILED DESCRIPTION

Polycrystalline silicon may be used in the production of electroniccomponents and solar panel construction. One conventional method ofproducing polycrystalline silicon is feeding a mixture comprisinghydrogen and silane (SiH₄) or a mixture comprising hydrogen and ahalosilane, such as trichlorosilane (HSiCl₃), is fed to a decompositionreactor containing a hot wire or hot substrate rods. This methodrequires a high amount of energy per unit of mass of produced silicon,and the silicon rods produced by this method need further processing tobe used in a silicon ingot growing process.

An alternate silicon production method is to feed a mixture comprisinghydrogen and silane or a mixture comprising hydrogen and trichlorosilaneto a fluidized bed containing silicon beads that are maintained at hightemperature. Decomposition of silane or trichlorosilane causes thedeposition of elemental silicon on the surface of the beads. Therefore,the silicon beads grow in size, and when large enough, are passed out ofthe fluidized bed reactor as a high purity silicon product. Incomparison to the substrate used in wire or rod substrate reactors,fluidized bed reactors allow for a much larger contact area between thebeads and the silicon-bearing gases in a heated chamber, enhancing thethermal decomposition of the silicon-bearing gases thereby forming highpurity elemental silicon on the surface of existing beads.

As described herein, the purity of the silicon produced using afluidized bed reactor may be preserved by constructing the reactor outof materials that do not contaminate the silicon product. In one suchembodiment, a fluidized bed reactor, or reactor, or reactor system usedfor the production of high purity silicon may be constructed out of amaterial that prevents or minimizes the contamination of thepolycrystalline silicon product due to the diffusion of impurities fromthe materials used to construct the reactor. In another embodiment, thereactor may be constructed or lined or coated with a material that isinert or resistant to the reactor feed gases and fluidized gases and theother gases or products that may be produced during the use of afluidized bed reactor.

In one embodiment, a fluidized bed reactor according to the disclosureherein may include an elongate chamber or column comprising one or moreinlet openings and one or more outlet openings. In one such embodiment,a fluidized bed reactor may include a bed of granular solid materials,such as a bed of silicon beads that can be used as seed beads to seed asilicon decomposition reaction during which the seed beads can increasein size because of the deposition of additional silicon on the surfaceof the seed beads. The seed beads with the added silicon product may beeventually removed from the reactor to recover the high purity siliconproduct. The seed beads may be “fluidized”, or suspended in the reactor,by injecting fluidizing gases into the reactor at sufficient velocitiesto agitate the beads. The fluidizing gases may be injected into thereactor through one or more inlet openings located around the reactorsuch as at the ends of the column and at the sides of the reactorcolumn. In one embodiment, the fluidizing gases or the silicon productmay be removed from the reactor through one or more outlet openings. Inone such embodiment, the reactor may be constructed or lined or coatedwith a material that is inert or resistant to the fluidizing gases usedto fluidize the bed of silicon beads.

A silicon-bearing gas may be injected into a fluidized bed reactor thatmay be constructed, lined or coated with a material that is inert orresistant to the silicon-bearing gas. In one embodiment, thesilicon-bearing gas may be trichlorosilane (TCS) that can be injectedinto the reactor at the same location or a location adjacent to thefluidizing gas. When heated, TCS decomposes in the reactor to formsilicon on the seed silicon beads thereby increasing the diameter of theseed silicon beads over time and producing the desired high puritysilicon product. One reason that the resulting polycrystalline siliconproduct is of high purity is because the reactor has been constructedout of a material that prevents or minimizes the contamination of thesilicon during decomposition. The resulting silicon product beads maythen be recovered from the reactor and used for the production ofsemiconductors and photovoltaic cells.

Methods for the production of high purity silicon may include the use ofa fluidized bed reactor configured to avoid reactor corrosion andprevent the contamination of a silicon product. In one embodiment, amethod of silicon production may include the conversion of metallurgicalgrade silicon (MGS) into a hydrohalosilane such as trichlorosilane(TCS); the purification of the hydrohalosilane, such as by distillation;and the decomposition of the hydrohalosilane back to silicon.

In one embodiment, the conversion of MGS into hydrohalosilane may beaccomplished by reacting silicon with silicon tetrachloride (STC),hydrogen and hydrogen chloride to form TCS and hydrogen. With referenceto FIG. 1, the following reactions may occur inside area 101:

3SiCl₄+2H₂+Si→4 HSiCl₃

SiCl₄+H₂→HSiCl₃+HCl

3 HCl+Si→HSiCl₃+H₂

In one embodiment, the result of the reaction in area 101 may be a mixof gases including TCS, STC, and H₂ that can be removed from area 101and then introduced in area 102 for purification by distillation.

The purification of TCS by distillation of the hydrohalosilanes mayoccur in area 102 as shown in FIG. 1. In one embodiment, the gas streamfrom area 101, including TCS, STC and other hydrohalosilanes, may beinjected into a distillation column in area 102 resulting in high purityTCS. Hydrogen may be recycled for use in area 101 after further purityremoval. The resulting TCS vapor is a silicon-bearing gas that may beinjected into a fluidized bed reactor that may be used for a silicondecomposition process in area 103.

Area 103 may comprise multiple elements for the conversion of TCS intohigh purity silicon. For example, area 103 may comprise one or more ofthe following: fluidized bed reactor, storage tank, evaporator, reactorheater, gas separator, granular separator, cyclone, heat recoverysystem, product recovery system and other devices and systems for theproduction of high purity silicon. The term hydrohalosalines refers toany silane species having one or more halide atoms and one or morehydrogen atoms bonded to silicon and includes, but is not limited tomonochlorosilane (H₃SiCl), dichlorosilane (H₂SiCl₂), trichlorosilane(HSiCl₃) and various chlorinated disilanes such as pentachlorodisilane.

In one embodiment, a silicon-bearing gas, such as a TCS vapor, may beused for the production of high purity silicon. The conversion of TCSinto high purity silicon may be accomplished using a fluidized bedreactor 200 as shown in FIG. 2, in which the following reaction mayoccur:

4SiHCl₃ 43 Si+3SiCl₄+2H₂ (thermal decomposition)

The fluidized bed reactor 200 used in the decomposition process mayinclude an elongate chamber or column 205 which includes a bed ofsilicon beads 210, which may be used to seed silicon a decompositionreaction. The beads 210 may be “fluidized” by initially injecting gases,such as fluidizing gases 215 from inlet 220 into the column 205 toagitate or fluidize the silicon beads 210. In one embodiment, thefluidizing gases 215 may include hydrogen and silicon tetrachlorideSiCl₄. In another embodiment, the fluidizing gas may be one or a mixtureselected from the group consisting of hydrogen, helium, argon, silicontetrachloride, silicon tetrabromide and silicon tetraiodide. In one suchembodiment, the fluidizing gases 215 may be injected into the column 205from several areas of the reactor 200 such as at the bottom or sides ofthe column 205, such as through inlet 220.

The fluidized bed reactor 200 may be heated by one or more heaters 240placed around or near the body of reactor 200. The heaters 240 may beradiant, conductive, electromagnetic, infrared or other type of heatersknown by those of skill in the art. In one embodiment, the surface ofthe reactor wall 250 may be textured, etched or sand-blasted in order toincrease the thermal emissivity or the thermal power transfer efficiencyof the reactor wall 250 and improve heating by the heater 240 of thecolumn 205 and the inside of the reactor 200.

In another embodiment, a heating device, such as heater 240, may be intotal or partial contact with the reactor wall 250. In yet anotherembodiment, the heater 240 may have no direct contact with reactor wall250. In one such embodiment, the heater 240 may be positioned outsidethe reactor wall 250 and configured as a group of cylinders partially orcompletely covering one or more outlet surfaces of the reactor 200. Instill another embodiment, the heater 240 may be configured to useradiation or a mix of direct heat conduction and heat radiation to heatthe silicon beads 210 and the silicon-bearing gases to a temperaturesufficient for the decomposition reaction.

In one embodiment, the fluidized bed reactor 200 may be heated duringthe production of high purity silicon to temperatures ranging fromapproximately 500° C. to approximately 1200° C. For example, thefluidized bed reactor 200 may be heated by the heaters 240 such that thesilicon beads 210, the silicon-bearing gases, and the fluidizing gases215 within the column 205 are heated to a temperature ranging fromapproximately 600° C. to 1100° C., or from 700° C. to 1000° C., or from700° C. to 900° C., or from 750° C. to 850° C., or from 800° C. to 1000°C.

The fluidized bed reactor 200 may be configured to withstand theconditions during the decomposition reaction including temperaturesranging from approximately 500° C. to approximately 1200° C. andinternal pressures ranging from approximately 50 mbar to approximately6000 mbar. For example, the fluidized bed reactor 200 as describedherein, may be constructed to withstand pressures of approximately up to50 mbar, 100 mbar, 200 mbar, 500 mbar, 750 mbar, 1000 mbar, 1500 mbar,2000 mbar, 2500 mbar, 3000 mbar, 3500 mbar, 4000 mbar, 4500 mbar, 5000mbar, 5500 mbar and 6000 mbar. In another embodiment, the fluidized bedreactor 200 may be contained within another structure or enclosureconfigured to support pressures ranging from approximately 50 mbar toapproximately 6000 mbar.

In one embodiment, one or more silicon-bearing gases, such as TCS, maybe injected into the reactor 200. For example, the silicon-bearing gasmay be injected into the reactor 200 through the inlet 220 into thecolumn 205. In one such embodiment, a silicon-bearing gas, like TCS,decomposes to form silicon on the beads 210, increasing the diameter ofthe beads 210 over time until they may become a silicon product bead212. In still another embodiment, the silicon-bearing gas may comprise agas which decomposes when heated to form silicon and is a gas or amixture of gases selected from the group of monosilane, disilane,trisilane, trichlorosilane, dichlorosilane, monochlorosilane,tribromosilane, dibromosilane, monobromosilane, triiodosilane,diiodosilane and monoiodosilane. In one embodiment, the high puritysilicon product beads 212 may be recovered from the reactor 200 near thetop of the column 205 at outlet 230 along with the effluent gas stream235 that may include hydrogen, STC, HCl, unreacted TCS andmonochlorosilane (MCS) and dichlorosilane (DCS).

In one embodiment, the concentration of the silicon-bearing gases in thefeed stream to the fluidized bed reactor 200 may range fromapproximately 20 mol % to 100 mol %. In one embodiment, the averagediameter of the fluidized silicon beads 210 may range from 0.5 mm to 4mm. In another embodiment, the average diameter of the silicon beads 210may range from 0.25 mm to 1.2 mm, or alternatively, 0.6 mm to 1.6 mm. Inone embodiment, the silicon beads 210 may remain in the reactor 200until a desired size is reached and the silicon product beads 212 areextracted from the reactor 200. In another embodiment, the time that thesilicon beads 210 may remain in the reactor 200 may depend on thestarting size of silicon beads 210. In one embodiment, the growth rateof the silicon beads 21 may depend, among other things, on the reactionconditions including gas concentrations, temperature and pressure. Theminimum fluidization velocity and design operational velocity may bedetermined by one of ordinary skill in the art based on various factors.The minimum fluidization velocity may be influenced by factors includinggravitational acceleration, fluid density, fluid viscosity, soliddensity, and solid particle size. The operational velocity may beinfluenced by factors including heat transfer and kinetic properties,such as height of the fluidized bed, total surface area, flow rate ofsilicon precursor in the feed gas stream, pressure, gas and solidstemperature, concentrations of species, and thermodynamic equilibriumpoint.

In one embodiment, one or more surfaces of the fluidized bed reactor 200may be made of a metal or a metal alloy. In one such embodiment, one ormore surfaces of the reactor 200 may include a metal or metal alloycapable of withstanding the reaction temperatures. For example, thereactor wall 250 may be constructed of iron based-alloys, such as:stainless steel alloys, chromium-nickel alloys, and nickel based alloysincluding nickel-chromium alloys and nickel-chromium-molybdenum alloys,which may optionally include manganese, molybdenum, silicon, cobalt,tungsten, etc., which would be apparent to those having skill in the artwith the aid of the present disclosure. In certain embodiments, themetal alloys may be chosen from: steel 1.4841, steel 1.4959, steel2.4856, steel 2.4819 or steel 2.4617. For example, the reactor wall 250may be configured to be thermoresistant to temperatures in the range ofapproximately 500° C. to 1,200° C. For example, the reactor wall 250 canbe constructed to tolerate temperatures ranging from approximately 500°C. to 600° C., or from 500° C. to 700° C., or from 600° C. to 800° C.,or from 800° C. to 900° C., or from 800° C. to 1000° C., or from 900° C.to 1100° C., or from 900° C. to 1200° C.

As shown by FIG. 2, the inside surface of the reactor wall 250 may bepartially or completely coated with a protective layer 260 to avoid orminimize the contamination of the product beads 212 by diffusion ofimpurities from the reactor 200 or the reactor wall 250. In one suchembodiment, the protective layer 260 may comprise materials that areinert or resistant to the conditions in the reactor 200, such as a metalor metal alloy capable of withstanding the reaction conditions withinthe reactor 200 and be compatible with the application of the protectivelayer 260. For example, the protective layer 260 may comprise materialsthat are resistant to heat, pressure, and corrosion by the fluidizinggases 215 or the silicon-bearing gases that are injected into thereactor 200.

In one embodiment, the fluidized bed reactor 200 may be lined with aprotective layer 260 comprising a ceramic material that is resistant tocorrosion or breakdown by the conditions in the reactor 200. In one suchembodiment, the protective layer 260 may comprise at least one of thefollowing materials: Alumina (Al₂O₃), Zirconium dioxide (ZrO₂) andZirconium dioxide—yttrium stabilized. In another embodiment, theprotective layer is a ceramic material made from a composition otherthan silicon-based or carbon-based. In yet another embodiment, theprotective layer 260 may comprise at least one of Alumina (Al₂O₃),Zirconium dioxide (ZrO₂) and Zirconium dioxide—yttrium stabilized incombination with at least one of polycrystalline silicon, siliconcarbide, silicon carbide coated graphite, silica, silicon nitride,tungsten carbide or molybdenum. In still another embodiment, thefluidized bed reactor 200 may include at least one of Alumina (Al₂O₃),Zirconium dioxide (ZrO₂) and Zirconium dioxide—yttrium stabilized incombination with one of: quartz, graphite, carbon fiber, or combinationsthereof.

FIG. 3 is close-up view of a cross section of a wall of a fluidized bedreactor as described herein with silicon beads 310 disposed therein. Inthe embodiment shown by FIG. 3, the reactor wall 350 comprises aprotective layer 360 and an adhesion layer 365 applied to the reactorwall 350 before the deposition of the protective layer 360. The adhesionlayer 365 may provide a substrate to which the protective layer 360 maybind or attach improving the durability and function of the protectivelayer 360. In one such embodiment, an adhesion layer 365 may comprise anickel based alloy with or without yttrium, particularly when iron basedalloys, such as Cr—Ni alloys, comprise the reactor wall 250.

In one embodiment, the protective layer 260 may comprise a coating whichhas a depth of approximately 3 to 1000 microns. In one such example, theprotective layer 260 has a depth ranging from approximately 5 to 900microns, 10 to 700 microns, 20 to 500 microns, 25 to 400 microns or 40to 300 microns. In another embodiment, the protective coating may have adepth of up to 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70,75, 80, 95, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800,or 900 microns.

The protective layer, such as protective layer 260 shown in FIG. 2, orprotective layer 360 shown in FIG. 3, can be formed or deposited by oneor more methods known by those of skill in the art including thermalprojection, chemical vapor deposition, physical vapor deposition,solgel, electrophoretic deposition and aerosol thermal spraying.

In one embodiment, the deposition of the protective layer 260 orprotective layer 360 may be followed by a thermal treatment. Forexample, the protective layer 260 may be annealed with a thermaltreatment of temperatures ranging from approximately 900° C. to 1,300°C. In one such embodiment, the thermal treatment may comprisetemperatures ranging from 900° C. to 1,000° C., or from 900° C. to1,100° C., or from 1,000° C. to 1,200° C., or from 1,000° C. to 1,300°C.

EXAMPLES

The specific examples included herein are for illustrative purposes onlyand are not to be considered as limiting to this disclosure. Thecompositions referred to and used in the following examples are eithercommercially available or can be prepared according to standardliterature procedures by those skilled in the art.

Example 1 Effects of Decomposition Conditions on Steel 1.4841

A. Nitrogen Gas

A sample of thermoresistant steel 1.4841 was subjected to decompositionconditions at 900° C. in the presence of silicon beads and a N₂ gasstream. After 100 hours under reaction conditions, the sample steel1.4841 was removed from the reactor and cooled to room temperature.

Cross sections of the sample steel 1.4841 were then prepared foranalysis with a scanning electron microscope (SEM). The SEM analysisrevealed that the steel had been corroded as evidenced by a silicidelayer extending into the metal to a depth of approximately 2 microns.

B. HCl and Hydrogen Gas

A sample of thermoresistant steel 1.4841 was subjected to decompositionconditions at 900° C. in the presence of silicon beads and HCl and H₂(5:1). After 100 hours under reaction conditions, the sample steel1.4841 was removed from the reactor and cooled to room temperature.

Cross sections of the sample steel 1.4841 were prepared as before forSEM analysis. The SEM analysis revealed a 50 micron silicide layer onthe metal resulting from corrosion of the metal substrate by thechloride. The SEM analysis also revealed the formation of chloridesincluding iron and chromium chlorides.

C. Steel with Cr₂O₃ Layer

A sample of steel 1.4841 was coated with a 50 micron layer of Cr₂O₃using chemical vapor deposition. The Cr₂O₃ coated steel was heated to900° C. in the presence of silicon beads and N₂ gas. After 100 hours,the Cr₂O₃ coated steel was cooled to room temperature. SEM analysisshowed the presence of silica and chromium on the surface of the steel,potentially due to the following reaction: 2Cr₂O₃+3Si→4Cr+3SiO₂.

D. Steel with Adhesion Layer and ZrO₂—Yttrium Stabilized ProtectiveLayer

A sample of steel 1.4841 was prepared with a nickel alloy adhesion layerto improve the adhesion of the ceramic layer. The nickel alloy adhesionlayer (NiCrAlY) was deposited using an atmospheric plasma spray process.Next, the sample was covered with a 100 micron ceramic coating ofZrO₂—yttrium stabilized, and heated to 900° C., as before, in conditionssimulating a fluidized bed reactor with silicon beads and in thepresence of HCl and H₂ (5:1). After 100 hours, the ZrO₂—yttriumstabilized coated steel was cooled to room temperature. SEM analysisshowed the steel 1.4841 with the ceramic coating of ZrO₂—yttriumstabilized resisted corrosion or degradation thereby minimizing oreliminating likely contamination of a silicon product. Moreparticularly, SEM data showed that there was no migration of aluminumoutside of the NiCrAlY adhesion layer. Likewise, there was no migrationof chromium, manganese, and nickel outside of the steel 1.4841 and theNiCrAlY adhesion layer. Additionally, the SEM analysis showed that onlya few particles of iron from the steel 1.4841 were present in the baseof the NiCrAlY adhesion layer and, there was no iron migration into theZrO₂—yttrium stabilized protective layer. Therefore, there was nocontamination of the ZrO₂—yttrium stabilized protective layer that wouldthreaten the purity of a silicon product.

E. Steel with Adhesion Layer and Al₂O₃ Protective Layer

As before, a sample of steel 1.4841 was prepared with a nickel alloyadhesion layer followed by the addition of an Al₂O₃ protective layer.The prepared sample was heated for 100 hours at 900° C. in a fluidizedbed reactor with silicon beads fluidized with HCl and H₂ (5:1). Aftercooling to room temperature, SEM analysis showed that the Al₂O₃protective layer prevented corrosion of the steel sample.

Example 2 Heat Transfer by a Radiant Heater

A. Untreated Stainless Steel

A stainless steel tube (AIS1316L) approximately 0.5 meters long and withan outside diameter of 21.3 mm and a thickness of 2.77 mm was used tomeasure radiation heat transfer. A radiation heater with an insidediameter of 40 mm was positioned around the steel tube withoutcontacting the surface of the steel tube. The steel tube and theradiation heater were insulated with 300 mm thick ceramic fiber. Astream of N₂ gas at mass flow rate of 15 Kg/h was passed along theinside of the steel tube horizontally. Thermowells were used to measurethe temperature of the radiation heater, the external temperature of thesteel tube at the inlet and outlet points of the N₂ gas stream, and thetemperature of the N₂ gas stream at the inlet and outlet points. Insteady state, the following temperatures were measured: N₂ inlettemperature=21° C.; N₂ outlet temperature=315° C.; outer wall tubetemperature at the inlet=569° C., and at the outlet=773° C. The thermalpower absorbed in the system was 1.325 W.

B. Sand-Blasted Stainless Steel

A stainless steel tube was prepared as previously described followed bysand-blasting the surface of the steel tube. After sand-blasting, thefollowing temperatures from the stainless steel tube were measured: N₂inlet temperature=20° C.; N₂ outlet temperature=445° C.; outer wall tubetemperature at the inlet=953° C., and at the outlet=1,055° C. With thesurface treatment, the thermal power absorbed in the system was 1.970 W.

The analysis showed that the thermal power transfer with thesand-blasting treatment was approximately 1.5 times greater that theuntreated steel tube. The thermal power transfer to the N₂ gas stream isdue to a combination of radiation from the heater to the outlet wall ofthe steel tube, conduction through the wall of the steel tube, andconvection from the inner wall of the steel tube to the N₂ gas stream.The surface treatment resulted in a decreased reflectivity of the steeltube and an increased ability of the steel tube to absorb heat, therebyincreasing the efficiency of heat transfer to the gas stream inside thesteel tube. An estimation of the new emissivity value was calculatedusing the combination of the radiation, conduction and convection heattransfer. Values of the theoretical thermal power transfer werecalculated in order to estimate the new value of the emissivity.

Not being bound by any particular theory, a calculation model using thefollowing equations was used:

Dittus-Boelter Equation:

Nu_(D) = 0.023 ⋅ Re_(D)^(0.8) ⋅ Pr^(n)$h_{int} = \frac{{Nu} \cdot k}{D}$

A grey body completely enclosed into another grey body equivalentconvection coefficient due to radiation:

$h_{ext} = {\frac{\sigma}{\frac{1}{ɛ_{1}} + {\frac{r_{1}}{r_{2}} \cdot ( \frac{1 - ɛ_{2}}{ɛ_{2}} )}} \cdot \frac{T_{1}^{4} - T_{2}^{4}}{T_{1} - T_{2}}}$

Universal coefficient of heat transfer at the inlet and at the outlet ofthe pipe:

$U = \frac{1}{\frac{r_{ext}}{r_{int} \cdot h_{int}} + {\frac{r_{ext}}{k} \cdot {\ln ( \frac{r_{ext}}{r_{int}} )}} + \frac{1}{h_{ext}}}$

The calculation model introduced the concept of mean logarithmicdifference of universal coefficients of heat transfer, and temperatures,because U and ΔT vary along the heat exchanger.

The emissivity value of the heater was ε=0.7 according to themanufacturer datasheet. The emissivity of stainless steel is 0.18 at500° C.

The values of the first test were implemented in order to adjust themodel, and in a second stage the values of the second test wereimplemented in order to obtain the new emissivity values through aniterative process. The sand-blasted stainless steel emissivitycalculated was 0.52. Hence, the radiation heat transfer ratio wasincreased approximately 3 times by the sand-blasting surface treatment.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

1. A method of producing high purity silicon comprising: injecting atleast one fluidizing gas into a fluidized bed reactor, wherein thefluidized bed reactor comprises: a chamber constructed of a metal alloy,the chamber including a gas inlet and an effluent outlet; a ceramicprotective layer deposited on an inside surface of the chamber, whereinthe ceramic protective layer comprises at least one of the following:alumina (Al₂O₃) and zirconium dioxide (ZrO₂); a bed of silicon beadsdisposed within the chamber; and at least one reactor heater; injectingat least one silicon-bearing gas into the fluidized bed reactor, heatingthe fluidized bed reactor with the at least one reactor heater to atemperature sufficient for thermal decomposition of silicon; andcollecting the high purity silicon that has been produced and depositedon the fluidized silicon beads; wherein the ceramic protective layer isresistant to corrosion by the at least one fluidizing gas or the atleast one silicon-bearing gas.
 2. The method of claim 1, wherein themetal alloy is an iron-based alloy selected from at least one of thefollowing: a stainless steel alloy and a chromium-nickel alloy.
 3. Themethod of claim 1, wherein the metal alloy is a nickel-based alloyselected from at least one of the following: a nickel-molybdenum alloyand a nickelchromium-molybdenum alloy.
 4. The method of claim 1, furthercomprising an adhesion layer positioned between the protective layer andthe inside surface of the chamber.
 5. The method of claim 4, wherein theadhesion layer comprises a nickel alloy layer, or anickel-chromium-yttrium alloy layer.
 6. The method of claim 1, whereinthe protective layer is deposited on the inside surface of the chamberby at least one of the following: thermal projection, chemical vapordeposition, physical vapor deposition, solgel, electrophoreticdeposition and aerosol thermal spraying.
 7. The method of claim 1,wherein an external surface of the chamber is sandblasted to improve thethermal power transfer efficiency of the chamber compared to anuntreated external surface.
 8. The method of claim 1, wherein thefluidizing gas is at least one of the following: hydrogen, helium,argon, silicon tetrachloride, silicon tetrabromide and silicontetraiodide.
 9. The method of claim 1, wherein the silicon-bearing gasis at least one of the following: monosilane, disilane, trisilane,trichlorosilane, dichlorosilane, monochlorosilane, tribromosilane,dibromosilane, monobromosilane, triiodosilane, diiodosilane andmonoiodosilane.
 10. The method of claim 1, wherein heating the fluidizedbed reactor with the at least one reactor heater to a temperaturesufficient for thermal decomposition of silicon comprises heating thefluidized bed reactor to a temperature of between approximately 500° C.to approximately 1200° C.
 11. The method of claim 10, wherein thefluidized bed reactor is heated to a temperature ranging fromapproximately 700° C. to approximately 900° C.
 12. A fluidized bedreactor for the production of high purity silicon, the fluidized bedreactor comprising: a chamber having dimensions to contain siliconparticles capable of being fluidized therein, the chamber having a wallconstructed of a metal alloy; a gas inlet in the chamber configured toreceive a gas to fluidize the particles inside the chamber, wherein thegas inlet is coupled to a source of silicon-bearing gas; a siliconparticle inlet configured for the addition of silicon particles into thechamber; an outlet in the chamber configured to allow the recovery ofthe high-purity silicon; an outlet configured to allow an effluent gasstream to leave the chamber; and a ceramic protective layer deposited onat least a portion of an inside surface of the chamber wherein theceramic protective layer comprises at least one of the following:alumina (Al₂O₃) and zirconium dioxide (ZrO₂); and wherein the ceramicprotective layer is configured to be resistant to corrosion by thefluidizing gas.
 13. The fluidized bed reactor of claim 12, wherein themetal alloy is an iron-based alloy selected from at least one of thefollowing: a stainless steel alloy and a chromium-nickel alloy.
 14. Thefluidized bed reactor of claim 13, wherein the metal alloy furtherincludes at least one of the following: manganese, molybdenum, silicon,cobalt and tungsten.
 15. The fluidized bed reactor of claim 12, whereinthe metal alloy is a nickel-based alloy selected from at least one ofthe following: a nickel-molybdenum alloy and anickel-chromium-molybdenum alloy.
 16. The fluidized bed reactor of claim15, wherein the metal alloy further includes at least one of thefollowing: manganese, molybdenum, silicon, cobalt and tungsten.
 17. Thefluidized bed reactor of claim 12, further comprising an adhesion layerpositioned between the protective layer and the inside surface of thechamber.
 18. The fluidized bed reactor of claim 17, wherein the adhesionlayer comprises at least one of the following: a nickel alloy layer anda nickel-chromium-yttrium layer.
 19. The fluidized bed reactor of claim12, wherein the ceramic protective layer deposited on the inside surfaceof the chamber is deposited by at least one of the following: thermalprojection, chemical vapor deposition, physical vapor deposition,solgel, electrophoretic deposition and aerosol thermal spraying.
 20. Thefluidized bed reactor of claim 12, wherein the chamber comprises anexternal surface and, wherein at least a portion of the external surfaceof the chamber has been treated to improve the thermal power transferefficiency of the chamber compared to an untreated external surface. 21.The fluidized bed reactor of claim 20, wherein the external surface ofthe chamber is sand-blasted to improve the thermal power transferefficiency of the chamber.
 22. The fluidized bed reactor of claim 12,wherein the chamber is constructed of a material configured to withstandinternal pressures ranging from approximately 50 mbar to approximately5000 mbar.
 23. The fluidized bed reactor of claim 12, wherein thechamber is constructed of a material configured to withstandtemperatures ranging from approximately 500° C. to approximately 1200°C.
 24. The fluidized bed reactor of claim 12, wherein the siliconparticles comprise silicon beads and the gas comprises at least one ofthe following: hydrogen, helium, argon, silicon tetrachloride, silicontetrabromide, silicon tetraiodide, monosilane, disilane, trisilane,trichlorosilane, dichlorosilane, monochlorosilane, tribromosilane,dibromosilane, monobromosilane, triiodosilane, diiodosilane andmonoiodosilane.
 25. The fluidized bed reactor of claim 12, furthercomprising at least one reactor heater, wherein the at least one reactorheater comprises at least one of the following: a radiation heater and aconduction heater.
 26. A fluidized bed reactor used in the production ofhigh-purity silicon, comprising: a chamber having dimensions to receivesilicon beads capable of being fluidized therein, the chamber having awall constructed of a metal alloy; a gas inlet in the chamber configuredto receive a fluidizing gas and a silicon-bearing gas to fluidize theparticles inside the chamber, wherein the gas inlet is coupled to asource of the fluidizing gas and the silicon-bearing gas; an outlet inthe chamber configured to allow a recovery of the high-purity silicon;an outlet configured to allow an effluent gas stream to leave thechamber; and a protective layer deposited on at least a portion of aninside surface of the chamber wherein the protective layer comprises atleast one of the following: alumina (Al₂O₃) and zirconium dioxide(ZrO₂).